A)
Netiquette - a set of rules for proper behaviour among users in a computer network
Cyberspace travelers - trevellers of the online world of computer networks
Cybernauts - a computer user who uses the internet
Newbies - any new participants in some activity
Bandwidth - a data transmission rate
Flame-bait - a message posted to a public Internet discussion group with the intent of provoking an angry response
Asterisk filler - a star-shaped character
Emoticon - a representation of a facial expression created by typing a sequence of characters
Flood - a massive amount of posts to a single newsgroup in a short period of time
Off-topic - a contribution that is not within the bounds of the current discussion
B)
Acronyms:
a. ATM - at the moment
b. IDK - I don't know
+. ASAP - as soon as posible
Slang:
a. fantabolous - fantastic & fabulous
b. threads - clothes
+. poor man's something - a worse version of something
Smiles:
a. :D - big smile
b. :( - sad
+. :) - happy
C)
Irina Kartseva's example.
BRB - Be Right Back
I use it it quite often because it saves your time when you need to go somewhere quickly.
Sources:
my head
wikipedia
thefreedictionary.com